Add Sandcastle Tide: the playable page and its place in the catalogue, on the rules in `sandcastle/tide.js` #149

A new game in the house style, mounted like the other forged games, built on the tested rules in sandcastle/tide.js.

Done looks like:

• [ ] sandcastle/index.html draws the beach grid, the rocks, the castle and the sea edge, and drives it only through sandcastle/tide.js; it adds no game rule of its own. Tap a sand square to build a wall, or move a cursor with the arrow keys and press Space; a Tide button (or Enter) calls the tide in. Input is ignored while the water spreads. Ambient motion only in the surf line rippling. The HUD shows what the rules state reports.
• [ ] Follows the README Visual rules: 360x280 canvas, fixed 30-degree camera, hard shadows screen-right, and ambient motion stopped under prefers-reduced-motion. Wet, dry, wall and rock squares differ by texture as well as colour. The canvas aria-label says how to play. The rules popup is at most four lines. There is a #mute button and no .mp3 is committed.
• [ ] Listed in GAMES in shell.js as { id: 'sandcastle', title: 'Sandcastle Tide', blurb: 'Pat down sand walls between tides and keep the castle dry through five high waters', kind: 'Strategy', input: 'Tap', accent: '#d8b25a', music: false, forged: true }. It has the matching <li> in the root index.html, a building in the town from town.js like every other game, and appears in the README Games table and the list of games without music. bun scripts/og.mjs has been run.
• [ ] rounds.mjs has a scripted round for sandcastle that reaches the end line. serve.test.mjs asserts that /sandcastle/ and /sandcastle/tide.js answer 200. playtest.test.mjs passes for the folder, and bun test passes.

Files: sandcastle/index.html, shell.js, index.html, README.md, serve.test.mjs, rounds.mjs, og/. Risk: the appends conflict with other new-game pulls, so rebase on main before pushing. Fits: a builder.